This invention relates to logarithmic amplifiers providing a logarithmic type transfer function for the input signal of the output signal over a wide dynamic range of input signals.
In conventional FM receivers, rectified signals derived from the 10.7 Mhz intermediate frequency signal are typically used to provide the gate trigger voltages for the signal strength meter, the stereo signal indicator, the muting circuit and the like. However, because the strength of the intermediate frequency signal is proportional to the electric field strength of the incoming received signal at the receiving antenna, and the voltage change necessary to change the triggering state is small in a narrow trigger region such as used in those circuits, it is important to have a logarithmic or similar amplifier which functions over a wide dynamic range of input signals. Furthermore, the range of adjustment for stereo sensitivity for stereo-monoral change-over and for the operating sensitivity of a muting circuit, both using a signal derived from the intermediate frequency signal, is narrow. For this reason, when the stereo sensitivity and the muting sensitivity are different, it is not feasible to use a single gate signal for both circuits.
As a result, various circuits have been proposed to overcome or avoid these problems. However, none of the prior art techniques have produced entirely satisfactory results. Prior art approaches have attempted to develop logarithmic amplifiers to enlarge the dynamic range over which a logarithmic transfer characteristic is achieved. One such amplifier uses a pair of diodes connected in antiparallel fashion in the feedback loop of a negative feedback amplifier. Another uses an automatic gain control amplifier circuit having its operating point and the saturation level appropriately adjusted to produce a logarithmic transfer characteristic for input to output signals. These amplifiers, however, can at most approximate a logarithmic transfer characteristic for a 40 to 60 dB signal range. The logarithmic approximation range of the first amplifier, for example, is 40 to 60 dB. The second described amplifier, on the other hand, has a simpler circuit construction, but its logarithmic approximation range is narrower.